Apparatus and method for decreasing bio-effects of magnetic gradient field gradients

ABSTRACT

A magnetic field generator includes a power source and a segmented or un-segmented coil connected to the power source to generate a time-varying magnetic field. Energy is applied to the coil so that the coil generates a time-varying magnetic field gradient with a magnitude of at least 1 milliTesla per meter and a rise-time of less than 1000 microseconds. The coil may be comprised of overlapping, non-overlapping or partially overlapping coil segments that may individually energized to further improve the operating characteristics of the coil to further decrease bio-effects in magnetic resonance imaging through the use of reduced pulse lengths and multi-phasic magnetic gradient pulses.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation application of U.S. patentapplication Ser. No. 13/917,712, filed Jun. 14, 2013, which is acontinuation-in-part of U.S. patent application Ser. No. 12/905,256,filed Oct. 15, 2010, which a continuation-in-part of U.S. patentapplication Ser. No. 12/488,105, filed Jun. 19, 2009, now U.S. Pat. No.8,154,286, which relies for priority on U.S. Provisional Application No.61/074,397 filed Jun. 20, 2008, the contents of which are incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention concerns an apparatus and a method for decreasingthe bio-effects of magnetic field gradients on tissue(s). Morespecifically, the present invention concerns an apparatus and a methodto decrease bio-effects on neurological tissue from magnetic fieldgradients, such as those experienced during Magnetic Resonance Imaging(“MRI”).

DESCRIPTION OF RELATED ART

As should be appreciated by those skilled in the art, MRI is atechnology whereby a magnetic field gradient is applied to tissue togenerate a response from the atoms and molecules in the tissue. Afterdetection of the response, and an image of the tissue is generated basedon that response.

The magnetic field gradient affects the tissue at an atomic and/ormolecular level, which is a desirable outcome of the magneticstimulation.

Unfortunately, magnetic field gradients also elicit undesirableresponses from tissue. In particular, neurological tissue generates,receives, and transmits electrical signals, and, as the laws of physicsdictate, changing magnetic fields can cause electrical field generationand consequent neurological stimulation. Accordingly, the effect ofmagnetic field gradients on neurological tissue is especially pronouncedby comparison with other tissue types.

A model for gradient-induced bio-effects was described by D R McNeal andJ P Reilly in the mid to late 1970's (D R McNeal, “Analysis of a modelfor excitation of myelinated nerve,” in IEEE Trans. Biomed. Eng.,23:329-337, 1976; and J P Reilly, “Electric and Magnetic field couplingfrom high voltage AC power transmission lines—Classification ofshort-term effects on people,” in IEEE Trans. on Power Apparatus andSystems, 97 (6): 2243-2252, 1978) based on classic membrane-excitabilityrelations (A L Hodgkin and A F Huxley, “A quantitative description ofmembrane current and its application to conduction and excitation innerve,” in J. Physiol., 117:500-544, 1952).

Subsequent elaborations of the model were disclosed by Reilly in“Sensory effects of transient electrical stimulation: Evaluation with aneuroelectric model,” IEEE Trans. Biomed. Eng., 32 (19) 1001:1011, 1985.

The model elaborated by Reilly invokes the telegraph equation firstapplied in the late 1800's to design trans-Atlantic cables.

As in underwater cables, neurons are organized in sections that areseparated by nodes of Ranvier. These nodes enforceresistance/capacitance (“RC”) equations that govern neuronal behavior,with time constants related to neuronal diameter and inter-nodaldistances.

Having provided a brief overview of the electrical characteristics,attention is now turned to magnetic field gradients.

At least with respect to MRI, magnetic field gradients have severalcomponents of particular interest. The first is the magnitude (referringto the maximum magnitude of the gradient field), which is typicallymeasured in milliTeslas (mT). Another important quantity is the gradientstrength, which is typically measured in milliTeslas per meter (mT/m).Further descriptors include the duration, rise-time and fall-time of thegradient, which are typically measured in milliseconds (ms) or inmicroseconds (μs). Another key descriptor is the slew rate (referring tothe magnitude of the change of the gradient field in time), which istypically measured in Teslas per meter per second (T/m/s).

As the laws of physics dictate, changes in magnetic fields result in thegeneration of electrical fields. Changes in magnetic field gradients intissue, therefore, also result in the formation of electric fields.

When studying neurological tissues, the changes in magnetic fieldgradients depolarize nerves, once a threshold is reached. This thresholdbecomes higher as the pulse duration becomes shorter. The relationshipbetween these variables follows a traditional, hyperbolic curve.

It is noted that experimental studies in humans (D. J. Schaefer, J. D.Bourland, and J. A. Nyenhuis, “Review of Patient Safety in Time-VaryingGradient Fields,” in J. Magnetic Resonance Imaging, 19:20-29, 2000) havevalidated the basic model for pulse durations as short as fiftymicroseconds.

In response to various MRI studies, regulatory agencies have codifiedthe strength-duration model into law, at least in Europe and the UnitedStates. (See Requirements for the Safety of MR Equipment for MedicalDiagnosis, IEC 60601-2-33 (with respect to Europe); see also Guidelinesfor Premarket Notifications for MR Diagnostic Devices, 21 C.F.R. §807.87(with respect to the US).)

As a result of these studies and as a result of the regulations that arebased on these studies, MRI manufacturers have attempted to designtriangular pulse sequences to conform to the limits prescribed by theReilly model, among others.

One such attempt is described in U.S. Pat. No. 6,198,282, which isdirected to an optimized gradient system for providing minimum-durationgradient pulses, the contents of which are incorporated herein byreference.

The prior art also includes evidence of interest in thestrength-duration curve from another direction, as manufacturers ofneuron-stimulators try to shape the pulse in order to increasestimulation. (See P. J. Maccabee, “Influence of pulse sequence, polarityand amplitude on magnetic stimulation of human and porcine peripheralnerve,” J. Physiol., 513:571-585, 1998).

As a result of numerous studies and advancements, those in the industrytraditionally have applied magnetic field gradients above a thresholdfor neuronal stimulation. To minimize neuronal stimulation, the durationof the magnetic field gradient has been extended. A longer durationhelps to minimize the impact of the magnetic field gradient on theneurological tissue. One immediately apparent downside to this approachis that the MRI diagnostic requires a substantial amount of time.

There appears to be wide acceptance in the MRI community in the beliefthat the attainment of high gradient slew rates (i.e., the change inmagnetic gradients over a short period of time) would be a welcomedevelopment for the industry.

As should be immediately apparent, one impetus for increasing slew ratesis to reduce scan time. It is postulated that, if a given MRI sequencerequires a certain number of pulses, then the application of shorterpulses would permit the MRI sequence to be completed in a shorter timeperiod (i.e., faster) than conventional techniques.

Among other benefits, saving time improves safety for unstable patients.Saving time also may reduce the cost of the MRI sequence.

A second incentive for increasing slew rate is to increase gradientfield strength, which improves spatial resolution. For a givenprescribed pulse sequence, the faster one can ramp up the magnetic fieldper pulse, the higher the gradient strength will be for the same overallscan time. Since the gradient strength is proportional to the spatialresolution of the MRI image obtained, a higher slew rate will result ina better spatial resolution. Increased spatial resolution may improvemedical diagnosis in some cases.

As discussed above, for manufacturers of MRI devices, limits have beenset for slew rates based on studies concerning the presence ofbio-effects due to neuronal stimulation. These limits have placed alimit on currently-available scanning technologies.

As also should be appreciated by those skilled in the art, thegeneration of a magnetic field gradient with a very small durationpresents technological challenges as well. Accordingly, there also hasbeen a technological barrier to decreasing the duration of the magneticfield gradient.

With respect to technological limitations, in some MRI devices, switchesare used to trigger the generation of a magnetic field gradient. Thetypes of switches traditionally used include Insulated-Gate BipolarTransistor (“IGBT”) and Metal Oxide Semiconductor Field EffectTransistor (“MOSFET”)-based devices. However, these traditional switchesare not capable of creating magnetic filed gradients with a sufficientlyshort duration to avoid neuronal stimulation.

Recent developments in switches offer a solution to the technologicalproblem experienced with prior art MRI devices.

Specifically, several generations of plasma physics experimentalistshave led the development of reliable solid-state switches andpulse-forming lines that are just now being introduced into thecommunity. (See H. Sanders and S. Glidden, “High Power Solid StateSwitch Module,” in International Power Modulator Symposium ConferenceRecord, pp. 563-566, 2004).

Those switches are capable of triggering pulses of ten-thousand amps inone microsecond, orders of magnitude higher than the IGBT andMOSFET-based systems currently employed in commercial gradient fieldgenerators for MRI systems (See D. A. Seeber, J. H. Hoftiezer, and C. H.Pennington, “Pulsed current gradient power supply for microcoil magneticresonance imaging,” in Magnetic Resonance Engineering, 15 (3): 189-200,2002).

It is axiomatic in the field of pulsed power technology that it is ofteneasier to close a switch than to open a switch. To take advantage ofthis physical parameter, solid-state switches may be combined withpulse-forming lines (“PFLs”).

PFLs, which are also known as Blumlein lines (named for the World-WarII-era inventor David Blumlein), are dielectric-filled transmissionlines that begin draining their charge when triggered by a solid-stateswitch. The transmission lines stop delivering current once thedielectric has been drained of charge. Blumlein lines can switch innanoseconds, and maintain currents for milliseconds. (K Gasthaus, “Amillisecond Blumlein line for the power supply of a high power laser,”in J. Phys. E: Instrum., 20:192-195, 1987). In order to deliver pulsesof varying widths, sets of PFLs may be triggered independently of oneanother into a common load.

In view of the foregoing, there exists a desire to apply higher magneticfield gradients to tissue in an MRI environment while avoiding adversebio-effects on that tissue.

SUMMARY OF THE INVENTION

Therefore, disclosed embodiments provide an apparatus and a method fordecreasing the duration of scan times for MRI. Disclosed embodimentsalso involve the application of magnetic field gradients with amagnitude greater than that traditionally employed. In one embodiment,the gradient slew rate may be up to hundreds of times greater thanpreviously applied. In other embodiments, the magnitude may be greater.

Disclosed embodiments may decrease bio-effects from magnetic fieldgradients by applying a magnetic field gradient to tissue within a timeframe below the response threshold for that tissue.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in connection with the drawingsappended hereto, in which:

FIG. 1 is a schematic illustration of a first embodiment of an apparatuscontemplated by disclosed embodiments;

FIG. 2 is a schematic illustration of a second implementation of anapparatus contemplated by disclosed embodiments;

FIG. 3 is a schematic illustration of a third implementation of anapparatus contemplated by disclosed embodiments;

FIG. 4 is a schematic illustration of a fourth implementation of anapparatus contemplated by disclosed embodiments;

FIG. 5 is a flow chart illustrating one method contemplated by disclosedembodiments;

FIG. 6 is a graph depicting the magnetic field gradient and durationtypical of a prior art MRI device;

FIG. 7 is a graph depicting the magnetic field gradient and durationcontemplated by disclosed embodiments;

FIG. 8 illustrates a first implementation of a segmented coilcontemplated by the disclosed embodiments;

FIG. 9 illustrates a second implementation of a segmented coilcontemplated by the disclosed embodiments; and

FIG. 10 illustrates a third implementation of a segmented coilcontemplated by the disclosed embodiments.

FIG. 11 demonstrates a conventional MRI system with a supplementarygradient coil set according to the disclosed embodiments.

DESCRIPTION OF THE INVENTION

Inventive concepts will now be described in connection with one or moreembodiments. It is intended for the embodiments to be representative ofthe invention and not limiting of the scope of the invention. Theinvention is intended to encompass equivalents and variations, as shouldbe appreciated by those skilled in the art.

Disclosed embodiments take advantage of a physiological loophole:according to accepted physiological models for ionic channel transport,bi-phasic pulses on the order of a microsecond are too fast for thenerve to change its polarization state, and are, therefore, effectivelyignored. Disclosed embodiments capitalize on this physiologicalloophole. According to the model elaborated by Reilly, this loopholeimplies that gradient field thresholds may be increased by factors offive or more above the usual hyperbolic model without triggering anybio-effects.

Disclosed embodiments also takes advantage of magnetic pulse deliverysystems that are more powerful than traditional systems used to deliverMRI pulses.

At least one disclosed embodiment may utilize solid-state switchesand/or PFL technologies that conform to the requirements of MRI gradientamplifiers. These requirements include increased repetition rates,impedance and trigger matching to MRI pulse programming consoles, andelectrical/acoustic noise shielding considerations. Downstreammodifications include the buttressing of gradient coils to handle thehigher electromagnetic (i.e., J×B) forces that will occur at highercurrent loads, especially in high resolution MRI systems with highstatic magnetic fields.

According to classic coil-design codes, eddy current shielding is likelyto be less of a problem than at current regimes, and reductions ininductance are predicted. The influence of eddy currents may further beminimized by acquiring signals at long TE, when eddy currents have dieddown, as contemplated by still another embodiment of the presentinvention.

At least one disclosed embodiment may make the bore of the cryostatsignificantly larger than the gradient coil, further reducing theinfluence of eddy currents. With respect to mechanical stabilizationrequirements, although the magnetic forces and torques (i.e., J×B) willbe high, the durations will be so short that the delivered impulse canbe handled with minimal mechanical stabilization over existing systems.

As a prelude to the discussion of the various embodiments of the presentinvention, a general discussion of MRI devices is now provided. Thisoverview is not intended to be limiting of the invention. It is providedmerely to assist with an understanding of the components of the variousembodiment of the present invention, as detailed below.

As a general rule, an MRI device includes a magnetic field generator,typically a magnetic coil and a radio frequency (“RF”) generator ortransmitter. The magnetic coil generates a magnetic field and the RFgenerator emits radio waves.

As should be appreciated by those skilled in the art, tissue isprimarily composed of water molecules, each of which contains hydrogenatoms. When a person's tissue is exposed to a strong magnetic field, thehydrogen atoms align with the direction of the magnetic field.Subsequently, the RF generator emits radio waves to the hydrogen atomswhile in the aligned state. Some of the energy from the radio waves isabsorbed by the hydrogen atoms in the water molecules, thereby alteringthe magnetic alignment of the hydrogen atoms. The altered magnetic stateis detected by the detector, which generates signals that are processedto form as an image.

With this overview in mind, reference is now made to FIG. 1. FIG. 1provides a schematic diagram of a first contemplated embodiment of a MRIdevice 10 designed according to this disclosure

The MRI device 10 includes a power source 19. The power source 19 may beany type of generator suitable for generating power to be provided tothe one or more of the components connected thereto. The generator mayprovide an alternating current (AC) or a direct current (DC), as shouldbe appreciated by those skilled in the art. The precise output of thepower source 19 is not critical to the operation of the presentinvention. Moreover, the power output, once generated, may be converteddifferent types (e.g., AC or DC) as required by individual components ofthe system.

In FIG. 1, the power source 19 is illustrated as providing power to eachof the various components of the MRI device 10 of the present invention.It is noted, however, that the depicted arrangement is meant to beillustrative only. As should be appreciated by those skilled in the art,the individual components of the MRI device 10 may receive power from acentralized source, such as the power source 19. Alternatively, thevarious components may receive power from alternative power sources.Accordingly, the depiction of a single power source 19 is not intendedto be limiting of the invention.

In addition, as detailed below, the MRI device 10 of the presentinvention is illustrated and discussed with reference to singlecommunication lines (or links) extending between the various components.The illustration of single communication lines is meant to simplify thediscussion and illustration of the various embodiments of the invention.As should be appreciated by those skilled in the art, there may bemultiple communication lines between the various components of the MRIdevice 10 as required for their operation. Moreover, the communicationlines are not intended to be limited to wired links. To the contrary,the communication lines may be wireless, as required or desired foroperation of the MRI device 10.

In one contemplated embodiment of the present invention, the powersource 19 may include a plurality of power sources 19, each of whichgenerates power with different characteristics, as required by thedevice(s) and/or components associated therewith.

As depicted in FIG. 1, power from the power source 19 travels in twodirections. Power from the power source 19 is conducted first along acommunication line 14 to a capacitor 16. Power from the power source 19is carried second along a communication line 18 to a processor 20.

The capacitor 16 may be of any size or type as would be appreciated bythose skilled in the art. As is its nature, the capacitor 16 stores acharge based on the power inputted from the power source 19. That chargeis eventually discharged, as discussed in greater detail below.

While FIG. 1 illustrates a single capacitor 16, a plurality ofcapacitors 16 may be employed without departing from the scope of thepresent invention. In one contemplated embodiment, the MRI device 10relies upon a plurality of capacitors 16 for its operation. As should beappreciated by those skilled in the art, plural sets of capacitors 16may be employed to generate successive magnetic field gradients.

In the second flow path, power from the power source 19 is provided tothe processor 20. The processor 20 may be of any type suitable forexecuting instructions, generating data, receiving data, storing data,and the like. In one contemplated embodiment, the processor 20 may be apersonal computer. In other embodiments, the processor 20 may be amainframe computer, a portable computer, a personal data assistant(“PDA”) or any other similar device. The exact design and functionalityof the processor 20 is not critical to operation of the presentinvention. Accordingly, the processor 20 may be of any type suitable forthe operation of the MRI device 10.

The capacitor 16 is connected, via a communication line 22, to a switch24. Accordingly, when the capacitor 16 discharges the stored charge, thestored charge passes through the communication line 22 to the switch 24.

The switch 24 is connected, via a communication line 26, to a coil 28.Accordingly, when the capacitor 16 is discharged, energy from thecapacitor 16 is passed to the coil 28, which generates a magnetic field30.

The coil 28 need not be a single coil. To the contrary, it iscontemplated that the coil 28 may include a plurality of coils 28, eachof which is capable of generating all or part of the magnetic field 30.Moreover, as should be appreciated by those skilled in the art, whereplural coils 28 are employed, the coils 28 need not be of the same typeor size. To the contrary, it is contemplated that, where plural coils 28are employed, they may be differ from one another to produce magneticfield gradients of differing magnitudes, periods, etc.

As also shown in FIG. 1, the MRI device 10 includes an RF transmitter32. As discussed briefly above, the RF transmitter 32 generates radiowaves 34. While one RF transmitter 32 is illustrated, it is contemplatedthat a plurality of RF transmitters 32 may be employed without departingfrom the scope of the present invention. Moreover, where plural RFtransmitters 32 are employed, they may be of different sizes, types,etc.

As illustrated, the magnetic field 30 and the RF waves 34 are directedat a tissue sample 36. While the tissue sample 36 may be a portion of anorganism, it may also be a complete organism.

After interaction of the magnetic field 30 and the RF waves 34 with thetissue 36, the tissue 36 generates a responsive signal 38 that isdetected by the detector 40. As should be appreciated by those skilledin the art, the signal 38 may encompass a multitude of different signalsfrom the tissue 36. The detector 40 detects the signals 38 and passesthe signals 38 to the processor 20 via the communication line 42. Theprocessor 20 receives and processes the signals 38 to generate an imagerepresentative of the composition of the tissue 36.

As should be appreciated by those skilled in the art, the processor 20may not be the device that processes the signals 38 to generate theimage of the tissue 36. To the contrary, the detector 40 may be combinedwith a suitable imaging device. In still another embodiment, the imagermay be a component separate from the processor 20 and the detector 40.Still further embodiments are contemplated to fall within the scope ofthe present invention.

With continued reference to FIG. 1, the MRI device 10 includescommunication line 42. Communication line 42 is illustrated as a centralbus that connects the processor 20 to the capacitor via communicationline 44, to the switch, via communication line 46, to the coil, viacommunication line 48, and to the RF transmitter, via communication line50. A central bus, however, is not required to practice the invention.To the contrary, multiple connections may be established between thecomponents of the MRI device 10 without departing from the scope of theinvention, as discussed above.

It is noted that the communication lines 14, 18, 22, 26, 42, 44, 46, 48,50 all may conduct data and/or power. The communication lines,therefore, are meant to illustrate multi-modal connections between thevarious components of the MRI device 10. As noted above, each of thecommunication lines 14, 18, 22, 26, 42, 44, 46, 48, 50 may be replacedwith one or more separate connections, as required or desired. Thecommunication lines 14, 18, 22, 26, 42, 44, 46, 48, 50 may beunidirectional or bidirectional as required or desired.

With respect to the communication lines 42, 44, 46, 38, 50, it iscontemplated that the processor 20 will provide operating instructionsto one or more of the components to which it is connected. The processor20, therefore, is contemplated to incorporate control functionality overone or more of the components, as should be appreciated by those skilledin the art. It is also contemplated that controls may be fed from onecomponent to another, as required or desired for operation of the MRIdevice 10.

FIG. 2 illustrates an MRI device 52, which is a second embodimentcontemplated by the present invention. Many of the components of the MRIdevice 52 are the same as illustrated and described in connection withMRI device 10 in FIG. 1. As a result, those components are provided withthe same reference numbers as the components in FIG. 1. Moreover, thedescriptions of these components is not repeated for the sake ofbrevity.

The MRI device in FIG. 2 differs from the MRI device 10 in FIG. 1 in atleast one respect. Specifically, the capacitor 16 and the switch 24 havebeen replaced with a pulse-forming line 54. The pulse forming line 54 isconnected to the power source 19 via communication line 56. The pulseforming line 54 is connected to the coil 28 via the communication line58. The pulse forming line 54 connects to the processor via thecommunication line 60.

As discussed above, a pulse forming line 54 is also known as a Blumleinline. Pulse forming lines 54 are transmission lines that begin drainingtheir charge in response to a triggering event, such as when triggeredby a solid-state switch. Pulse forming lines 54 also are referred to aspulse forming networks. A pulse forming network (“PFN”) accumulateselectrical energy over a predetermined period of time and releases theelectrical energy in the form of a square pulse in a relatively shortperiod of time, depending upon the materials that make up the PFN 54.PFNs 54 also may be engineered to provide pulsed power. A PFN 54 may becharged by a high voltage power source 19 and then rapidly discharged(possibly via a high voltage switch).

The pulse forming line 54 may be a single line or may be a plurality oflines combined together. The pulse forming line 54 also may be a pulseforming network 54, as discussed above. The exact composition andconstruction of the PFN 54 is not critical to operation of the presentinvention.

FIG. 3 illustrates a third embodiment of an MRI device 62. Thisembodiment is similar to the MRI device 52 illustrated in FIG. 2. Inthis third embodiment, however, a switch 64 has been added between thepower source 19 and the pulse forming line 54. As illustrated, the powersource is connected to the switch via a communication line 66. Theswitch 65, in turn, is connected to the processor 20 via thecommunication line 68. As noted above, the switch 65 may be employed totrigger the pulse forming line 54 to release its energy to the coil 28.

FIG. 4 illustrates a fourth embodiment of an MRI device 70. This fourthembodiment is a modification of the MRI device 62, which is illustratedin FIG. 3. Here, a capacitor 72 has been inserted between the switch 64and the power source 19. The capacitor 72 connects to the power sourcevia the communication line 74. The capacitor 72 connects to theprocessor via the communication line 76. In this embodiment, it iscontemplated that the capacitor 72 will discharge power to the switch64, which will discharge power through the pulse forming line 54 to thecoil 28, as illustrated.

With respect to the embodiments illustrated in FIGS. 1, 2, 3, and 4, andfor purposes of the present invention, the connection between the powersource 19 and the coil 28 is considered to be a controlled communicationline. As a result, the embodiments provide variations for contemplatedconstructions of that controlled communication line.

FIG. 5 illustrates a method 78 contemplated by the present invention.The method 78 starts at 80 and ends at 88. Following the start 80, at82, a magnetic filed gradient with a maximum of 1 milliTesla per meter(mT/m) is generated. At 84, the magnetic field gradient is maintainedfor a least about 1 microsecond and up to about 10 microseconds. At 86,the magnetic field gradient is changed in a time frame small enough tofail to solicit a response from neurological tissue. The method 78 endsat 88.

It is contemplated by the present invention that the application of amagnetic field gradient of no more than 1 mT/m will avoid triggering abiological response from neurological tissue. It is also contemplatedthat application of the magnetic field gradient for less than about 10microseconds also will establish suitable conditions to avoid triggeringa biological response from neurological tissue.

FIG. 6 is a graphical illustration of a single magnetic field gradientpulse according to the prior art. As illustrated, the duration T1 of themagnetic field gradient pulse exceeds a neurological response time T2for neurological tissue. In addition, the magnitude of the magneticfield gradient pulse is less than about 0.2 mT/m.

FIG. 7 provides a graphical illustration of a magnetic field gradientpulse generated according to the present invention. The pulse durationT3 is less than the pulse duration T1 according the practice in theprior art. Also, the pulse duration T3 is less than the neurologicalresponse time T2. In keeping with the invention, the pulse duration T3is less than about 10 microseconds, which is considered to beappropriate for avoiding a neurological response. In one contemplatedembodiment, the pulse duration T3 is about 1 microsecond.

A comparison between FIGS. 6 and 7 also illustrate one further aspect ofthe present invention. Specifically, the magnetic field gradient asgenerated by the prior art is about 5 times weaker than the magneticfield gradient generated by the MRI device 10 of the present invention.

As discussed, the magnetic pulses are created by releasing electricalcharge stored in capacitors and/or transmission lines into coils and/ortransmission lines near a body part. The coils may include dielectricand/or ferrite materials which assist in the shaping of the magneticpulses. Ferrite materials may be used to construct complex dynamicelectromagnetic fields for circulators and other circuit elementsemployed in microwave transmission and receivers (as disclosed by AnsoftCorporation, in Microwave Journal, June, 1996).

Moreover, as disclosed in U.S. patent application Ser. No. 19/488,105and entitled “Apparatus and Method for Decreasing BioEffects of MagneticField Gradients”, (incorporated by reference), and discussed above, aplurality of coils may, thus, be used to generate magnetic fieldgradients. In accordance with further disclosed embodiments, variousadvantages and applications may be provided by this generation.

For example, although the rise-time of a magnetic field produced by acoil is inversely proportional to the inductance of the coil, andproportional to the voltage applied to the coil, it may be difficult toimplement voltages of arbitrary magnitude due to safety requirements orlimitations of insulators. Therefore, in accordance with disclosedembodiments, it may be useful and beneficial to segment a coil intoseparate sections or segments, each one energized by a separate powersupply. As a result of such an implementation, the voltage requirementfor each power supply may be reduced. Additionally, the inductance foreach coil segment may be less than the sum of all the segments, enablingshorter rise- and fall-times as compared to the case in which one powersupply was to energize one large coil.

In accordance with disclosed embodiments, the coil segments may bephysically overlapping, for example, in the case where each winding orlayer on a single mandrel or substrate is attached to a different powersupply, as illustrated in FIG. 8.

As shown in FIG. 8, a plurality of coils 1, 2, 3 may overlapping withrespect to a mandrel or substrate 4. These coils 1, 2, 3 may be eachenergized by separate amplifiers.

Alternatively, the coil segments may be physically separate, as in thecase where the coil segments do not overlap, as illustrated in FIG. 9.As shown in FIG. 9, a plurality of coils 5, 6, 7 are not overlappingwith respect to a mandrel or substrate 8, but may each still energizedby separate amplifiers.

As a further alternative, the coil segments may be partially overlappingas shown in FIG. 10 wherein a plurality of coils 13, 15, 17 arepartially overlapping with respect to a mandrel or substrate 19, andwhich may be each energized by separate amplifiers (e.g., a single powersource connected to separate power amplifiers to provide differentiatedpower levels). It should be understood that the term “power source”encompasses both independent power sources and a single power sourcecoupled to a plurality of power amplifiers to provide power at differentpower levels, i.e., differentiated power levels, that may be viewed asdifferent power sources.

It should be appreciated that the times at which each coil segment maybe activated may be the same or may differ in order to produce differentmagnetic field gradients.

It should also be understood that switches may be employed tofunctionally separate amplifiers to drive the separate coils, and thatat times these or other switches may be configured so that some or allof the separate coils may be energized by a functionally singleamplifier.

Another factor affecting rise time is the skin effect, in which asubstantial portion of high-frequency currents traveling through aconductor are forced near the surface of the conductor. Since thecross-sectional area of the conducting channel is reduced by the skineffect, and the resistance of the conductor is inversely proportional tothe cross-sectional area of the conducting channel, the resistance ofthe conductor is effectively increased. Said increased resistance isunwelcome, since it leads to energy loss. In the electrical designcommunity, the energy loss at high frequencies has been addressedthrough the use of interleaved conductor configurations (also known aswound or “Litz” wire), which effectively increases the cross-sectionalarea of the interleaved conductor at high frequencies as compared to asolid wire of similar total diameter. The short rise- and fall-times ofthe contemplated embodiment produce high frequency components, which asdiscussed above can lead to increased resistance. The rise-time andfall-time of the current is governed by the equation dI/dt=(V−R*I)/L,where “I” represents the coil current, “t” is time, “V” is the voltageapplied to the coil, “R” is the coil resistance and “L” is the coilinductance. Therefore, reducing the resistance through the use ofinterleaved conductors in some or all of the coils has a beneficialeffect on rise- and fall-time.

Thus, this disclosure enables the delivery of very short multi-phasicmagnetic gradient pulses for magnetic resonance imaging. The disclosurealso provides a method of decreasing bio-effects in magnetic resonanceimaging through the use of reduced pulse lengths and multi-phasicmagnetic gradient pulses.

FIG. 11 demonstrates a conventional MRI system with a supplementarygradient coil set, combined as MRI device 51. The conventional MRI isshown with components: permanent magnet 53, native gradient coil 55,radiofrequency coil set 57. The drawing of the device 51 illustrated inFIG. 11 employs symmetry about the patient for the above components, asis typical for MRI systems. It is understood that additional equipment(for example, digitizers and amplifiers), not illustrated in FIG. 11,may be required to operate the illustrated components of a conventionalMRI in a pulse sequence to collect images of a body part in a patient 59resting on a table 61.

A supplementary coil set 63, including gradient and/or radiofrequencyantenna elements) may be arrayed near the patient's pelvis (close to theprostate 65) in order to generate fast high pulses for improving spatialresolution and signal-to-noise ratio of the prostate. Additionalradiofrequency coils and other ancillary equipment needed to operate thesupplementary gradient coil set (for example, amplifiers and digitizers)are understood to potentially be required, and therefore are omitted inFIG. 11.

It should be understood that the magnetic field generated may haveeither a rise-time or fall-time of less than 100 microseconds or,alternatively, less than 250 microseconds. The magnetic gradient mayhave a maximum gradient field strength of more than 10 mT/m, 50 mT/m,100 mT/m, 200 mT/m, 1 T/m. The magnetic gradient may have a maximum slewrate of more than 200 T/m/s, 500 T/m/s, 1000 T/m/s, 1000 T/m/s, or 1million T/m/s.

The magnitude of the magnetic field may be, for example, more than 50mT, 100 mT, 200 mT, 300 mT, 400 mT, or 500 mT.

A pulse sequence with multiple fast repetitions may be generated,wherein the repetitions increase the signal-to-noise ratio per pixel. Inaccordance with at least one embodiment, at least one coil is introducedinto a magnetic resonance system to improve visualization of a bodypart.

Other aspects of the present invention should be apparent to thoseskilled in the art based on the discussion provided herein.

What is claimed is:
 1. A magnetic field generator for imaging, thegenerator comprising: a power source; and at least one coil connected tothe power source to generate a magnetic field, wherein the magneticfield has a rise- and fall-time selected so as to be too short to elicita neurological response in tissue, and wherein the magnetic field has aslew rate of more than 200 T/m/s.
 2. The generator of claim 1, whereinthe magnetic field has either a rise-time or fall-time of less than 100microseconds.
 3. The generator of claim 1, wherein the magnetic fieldhas either a rise-time or fall-time of less than 250 microseconds. 4.The generator of claim 1, wherein the magnetic field has a gradientfield strength of more than 10 mT/m.
 5. The generator of claim 4,wherein the generator produces a pulse sequence with multiple fastrepetitions, wherein the repetitions increase the signal-to-noise ratioper pixel.
 6. The generator of claim 1, wherein the magnetic field has aslew rate of more than 500 T/m/s.
 7. The generator of claim 1, whereinthe magnetic field has a slew rate of more than 1000 T/m/s.
 8. Thegenerator of claim 1, wherein the magnetic field has a slew rate of morethan 1000 T/m/s.
 9. The generator of claim 1, wherein the magnetic fieldhas a slew rate of more than 1 million T/m/s.
 10. The generator of claim1, wherein the magnetic field has a gradient field strength of more than50 mT/m.
 11. A method of imaging tissue using a magnetic fieldgenerator, wherein the generator comprises a power source and at leastone coil, the method comprising: generating a magnetic field that has arise- and fall-time selected so as to be too short to elicit aneurological response in tissue, wherein the magnetic field has a slewrate of more than 200 T/m/s.
 12. The method of claim 11, wherein themagnetic field has either a rise-time or fall-time of less than 100microseconds.
 13. The method of claim 11, wherein the magnetic field haseither a rise-time or fall-time of less than 250 microseconds.
 14. Themethod of claim 11, wherein the magnetic field has a gradient fieldstrength of more than 10 mT/m.
 15. The method of claim 11, wherein themagnetic field has a slew rate of more than 500 T/m/s.
 16. The method ofclaim 11, wherein the magnetic field has a slew rate of more than 1000T/m/s.
 17. The method of claim 11, wherein the magnetic field has a slewrate of more than 1000 T/m/s.
 18. The method of claim 11, wherein themagnetic field has a slew rate of more than 1 million T/m/s.
 19. Themethod of claim 11, wherein a pulse sequence is generated with multiplefast repetitions, wherein the repetitions increase the signal-to-noiseratio per pixel.
 20. The method of claim 11, wherein at least onesection of a pulse sequence used to collect data about a body part isrepeated more than 100 times per second.
 21. The method of claim 11,wherein at least one section of a pulse sequence used to collect dataabout a body part is repeated more than 100 times per second.
 22. Themethod of claim 11, wherein at least one section of a pulse sequenceused to collect data about a body part is repeated more than 1000 timesper second.
 23. The method of claim 11, wherein at least one section ofa pulse sequence used to collect data about a body part is repeated morethan 10000 times per second.
 24. The method of claim 11, wherein the atleast one coil is introduced into a magnetic resonance system to improvevisualization of a body part.
 25. The method of claim 11, wherein themagnitude of the magnetic field is more than 50 mT.
 26. The method ofclaim 11, wherein the magnitude of the magnetic field is more than 100mT.
 27. The method of claim 11, wherein the magnitude of the magneticfield is more than 200 mT.